The invention relates to a gas-operated spring for the mutual support of two oscillating elements moving relative to one another. First and second sensor-controlled devices bring about the venting of excess gas from the spring or the feeding in of additional compressed gas to the spring through the activation of valves. The first device compensates for vibration induced pressure changes in the spring, and the second device compensates for changes in the relative position of elements supported by the spring. The function of the second device is cancelled by a frequency filter upon the introduction of vibrations with a frequency greater than 1 Hz.
Such a gas-operated spring is known from U.S. Pat. No. 4,761,020. It makes use of a frequency filter in order to prevent the functions of the two devices adversely affecting one another. The excess gas under pressure is blown out of the interior space into the atmosphere.
The first device which is provided to compensate for vibration-induced pressure changes in the gas-operated spring, may be used when a gas-operated spring is used in the area of the wheel support of a motor vehicle, where such changes in pressure can be caused by driving over a cobblestone pavement. They produce a change in the forces supporting the car body and become noticeable in the same in the form of humming vibrations. There is therefore the desire to compensate for corresponding pressure changes as completely as possible. From a technical point of view, the problems in this connection are largely solved, so that, with the present state of the art, it is possible even in such problem cases to ensure that the internal pressure of the gas-operated spring is largely kept constant at a particular level and in this way to prevent changes in the support forces exerted on the car body. Aside from the first devices described above, the previously known gas-operated springs have second devices, which are intended to ensure a constant relative position of the elements supported by the gas-operated spring.
They have a function opposite to that described above, inasmuch as, for example, the load-induced lowering of a vehicle can be compensated for only if additional compressed air is supplied to the gas-operated spring until the original level is regained. Automatically associated with this is a pressure increase in the gas-operated spring. At the same time, a deviation in pressure from the level originally present is selectively brought about. It is therefore entirely conceivable that the functions of the first and the second devices will adversely affect one another. For this reason, a frequency filter is connected upstream from the second device in the case of the gas-operated spring of U.S. Pat. No. 4,761,020 . By means of this filter, the function of the second device is canceled in the event of positional changes in the supported object of a frequency greater than 1 Hz. This solution of the problem is still not very satisfactory for some technical applications.